Parkinsonism or Parkinsonian syndromes are a group of progressive, multicentric neurodegenerative disorders whose main features are tremor at rest, rigidity, bradykinesia and postural instability. Parkinson's disease (PD) is the most common form of Parkinsonism and the second most common neurodegenerative disorder after Alzheimer's disease. In industrial countries, the prevalence of PD has been estimated at approximately 0.3% of the general population, the elderly being the most at risk (4% of the population over 80 are estimated to be affected). The mean age of onset is around 60 years, although early onset (as young as 20 year old) can occur [1].
PD is often classified as a movement disorder. Rest tremor is the most common and usually among the earliest symptoms to develop. Bradykinesia also usually appears in the early stages with difficulties performing tasks such as writing or getting dressed. Rigidity occurs and progresses to stiffness and resistance to movement of the whole body, reducing the ability to move. In the late stages, the disease progresses to postural instability, leading to impaired balance and frequent falls. Other motor symptoms such as gait or swallowing disturbances can arise. If not treated, motor symptoms can lead to the patient being bedridden after an average of ten years [2, 3].
In later stages of the disease, PD gives rise to many non-motor symptoms which vary greatly individually. Disability is then greatly worsened by the development of autonomic and neuropsychiatric disturbances. Disorders of speech, cognition, mood, behavior, and/or thought will develop, leading eventually to dementia. Other common symptoms include sensory, sleep and emotional problems. Those disorders decrease the life expectancy of the individual affected and the mortality ratios are around twice those of people without PD [2-4].
PD is an idiopathic disease and its pathophysiology also remains poorly understood [4]. However, at least 5% of PD cases can be attributed to genetic variations. Mutations within genes such as SNCA (alpha-synuclein), PRKN (parkin), LRRK2 (leucine-rich repeat kinase 2), PINK1 (PTEN-induced putative kinase 1), DJ-1 and ATP13A2 and eleven gene loci (PARK1-PARK11) have been associated with familial PD [5]. Apart from genetic factors, many environmental risk factors have been proposed to be involved in the onset of PD but none with undisputed evidence. The most frequently replicated risk factor is exposure to pesticides or herbicides such as Agent Orange. On another hand, smoking and caffeine consumption seem to protect individuals from PD [1].
The pathophysiology of PD is characterized by three features [4]:
(i) A synucleinopathy characterized by the abnormal accumulation of alpha-synuclein protein into inclusions called Lewy bodies in the brain. The distribution of the Lewy bodies throughout the brain varies from one individual to another but is often directly associated with the expression and degree of the clinical symptoms.
(ii) A dopaminergic activity deficiency due to the death of dopamine-generating cells in the substantia nigra, a region of the midbrain. This results in a loss of muscle movement and tone control, leading to the motor symptoms of PD.
(iii) Degeneration of NANC (non-adrenergic, non-cholinergic), serotonergic and cholinergic neurons also occurs in later stages of the disease, leading to the non-motor symptoms of PD.
As no biological tests are available, diagnosis of PD is mainly based on observation of clinical symptoms and exclusion of other disorders with similar clinical features [3]. Postmortem confirmation is required for a definitive diagnosis. Neurological examination by neuroimaging can be useful to detect changes in dopaminergic neurons and to rule out other diseases. Positive therapeutic response to levodopa is another diagnosis criterion. Once the diagnosis made, the progression and severity of the disease is rated using a stages scale such as the Unified Parkinson's Disease Rating Scale.
The most widely used treatment, especially at earlier stages, is the dopamine precursor levodopa (L-DOPA) [6]. The drug brings the lacking neurotransmitter to the dopaminergic neurons, thus decreasing motor symptoms. However, most of the drug is metabolized before to reach the blood brain barrier, causing a variety of side effects, especially dyskinesia [7]. To prevent this phenomenon, L-DOPA is therefore usually given in combination with carbidopa or benserazide (peripheral dopa decarboxylase inhibitors) and often also with catechol-O-methyl transferase inhibitors such as entacapone. These drugs aim at preventing L-DOPA metabolism before to reach the brain, enhancing the activity of the drug [6]. Although less effective at improving motor symptoms, dopamine agonists such as pergolide, cabergoline, apomorphine or lisuride and monoamine oxidase-B inhibitors (involved in the catabolic breakdown of dopamine) such as selegiline or rasagiline are commonly used at early stages of the disease. Although less effective, they may be useful at delaying the use of levodopa and thus the onset of dyskinesia [7].
Other drugs such as anticholinergics and nicotinic acetylcholine receptor may be useful but their efficacy for PD remains to be confirmed [7]. Current research also focuses on neuroprotective treatments, but none of them provided evidence of improved degeneration. They target apoptosis (omigapil, CEP-1347), glutamate receptors, adenosine A2A receptor, calcium channels (isradipine), growth factors (GDNF), alpha-synuclein and inflammation [8]. Ongoing pharmaceutical research has shown a growing interest on gene therapy and neural transplantation [8].
PD remains so far an incurable disease and no effective disease-modifying treatment has been discovered yet. Therefore, current treatments aim at relieving symptoms and alleviate the slow progression of the disease.